A hydrodynamic model of the west coast of Scotland with coupled sea lice dispersion

In order to assess the risk that wild salmon and sea trout will be harmed by parasitic sea lice emanating salmon farms a three-dimensional hydrodynamic and biological model of the West Coast of Scotland has been developed. The model uses TELEMAC-3D-WAQTEL and the computational domain extends from the Mull of Kintyre in the South to Cape Wrath in the North and includes all main islands of the West Coast. The model was successfully validated against observed hydrographic data (water levels and currents) and was found to provide a reasonable description of salinity and temperature levels. In an integrated biological model, virtual particles were constructed within the framework of the open-source particle-tracking code OpenDrift. These were released at each farm site and allowed to disperse into the marine environment. Each particle is a “super-individual”, representing a number of sea lice larvae. The biological effects of sea lice production, maturity and mortality rates, salinity avoidance, temperature preference and phototactic vertical swimming behaviour (diel migration) were included. Results show that infective lice copepodids accumulate along tidal and salinity fronts, at the mouths of sea lochs and along shorelines, in different places according to the neap/spring tidal cycle and provide an indication of infestation risk to migrating wild fish.